The invention relates to the field of powder coating. More particularly, the invention relates to a method and apparatus for powder coating. The invention also relates to the production of coatings with multiple layers, combined coating and printing operations on a variety of substrates, and in particular combined coating and printing operations.
The coatings industry has long used utilized liquid coating processes and apparatus, with coatings being applied by spraying or rolling upon a target object. This technology has been used for functional coatings, such as for pipe and reinforced steel bar (rebar), for example.
For several decades, however, the coating industry has increasingly adopted powder coating technology in place of conventional liquid coatings. The preference for powder has occurred to realize environmental and other advantages of powder coatings.
Instead of being suspended in a liquid medium, such as a solvent or water, and applied as a liquid to an article to be coated, a powder is applied dry, i.e., in a granular form. Consequently, a powder coating contains no solvents and emits essentially no volatile organic compounds (VOC's). In addition, venting, filtering, and recovery of solvents are avoided with powder coating.
Powder coating materials are typically applied to conductive substrates by means of spray guns, using an electrostatic deposition technique. The powder, entrained in an airflow and corona or tribo-charged before application, is directed at the conductive substrate.
Other electrostatic deposition techniques are also known, such as that using a fluidized bed and that using a cloud chamber, although electrostatic spraying is the dominant technique used in the industry.
After a substrate is coated according to known electrostatic deposition techniques, the powder coating is cured on the substrate, most typically using an oven or other energy source where the powder is heated to form a final film, or by exposure to chemical vapors. It is an objective to create a continuous final film on the substrate.
However, when relying upon present-day electrostatic apparatus and methods of using such apparatus, uneven coatings can result, which can then require the application of an undesirably thick coating to ensure that the substrate is completely coated in view of such unevenness or non-uniformities.
The application of charged powders or toners to substrates or receivers by means of an electric field is also performed by processes commonly known in electrography and particularly in photocopying technology, laser printer technology, or ionography (these application processes are elucidated in, for example, L. B. Schein, “Electrography and Development Physics”, Laplacian Press, 1996, the disclosure of which is incorporated herein by reference).